Telephone hybrid
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A telephone hybrid is a relatively simple electronic device used to connect a telephone system to regular audio circuits. These are normally used in radio stations (and sometimes TV stations and broadcast networks) to connect callers into the airchain, so that conversations may be broadcast.
The main principle at work is impedance matching, although the hybrid must also block the 48 V direct current of the POTS telephone line from the normal audio side. Some inexpensive designs connect to the handset cord, with a button to activate either the handset or the hybrid. These only cost around US$100, need no power, and don't even need to block DC as none goes through the handset. More expensive versions can cost thousands of dollars or more, but can handle multiple lines, and may connect to a computer so that a producer can keep up with who is on which line, and what goes on the air next. Many also perform audio level compression, and even full digital signal processing to make the bandwidth-limited telephone audio sound more acceptable on-air.
Digital hybrids are even used for broadcasting over standard telephone systems, using a special unit with DSP audio data compression and decompression at each end. Audio bandwidths up to 15 kHz (full FM broadcast quality, and the range of most human hearing) can be achieved this way, along with slow auxiliary data that can remotely trigger relays on the mixing console or other automated equipment back at the station. Compression is often via MPEG, particularly now MPEG-4.
